Arbitrary fibonacci

#include <iostream>
#include <math.h>
#include <gmpxx.h>

using namespace std;

mpz_class fibo(unsigned long fib)
{
    mpz_class i = 1;
    mpz_class j = 0;

    for(size_t k = 1; k <= fib; k++)
    {
        mpz_class t;
        t = i+j;
        i = j;
        j = t;
    }

    return j;
}

int main()
{
    mpz_class fib, modValue = 0, test;
    unsigned long m = 0, n = 0;

    while(cin >> n >> m)
    {
        if( n >= 0 && n <= 2147483647 && m >= 0 && m < 20)
        {
            fib = fibo(n);
            mpz_ui_pow_ui (modValue.get_mpz_t(), 2, m);

            test = fib % modValue;
            cout<<test<<"\n";
        }
    }

    return(0);
}